The overall goal of this work is to study the autonomic control of cardiac excitation, and atrioventricular conduction. Project one is a study of the selective vagal control of SA and AV nodal functions. A new technique has been devised for stimulating, in vivo, the postganglionic vagal fibers that selectively innervate each node. This permits, for the first time, the collection of data related to the function of each node where that data is not contaminated by any functions of the other node. For example, stimulation of the cervical vagus profoundly alters heart rate as well as AV nodal conduction time. Changes in heart rate also strongly influence AV conduction, however, such that to study the pure vagal effect on AV conduction it has before been necessary to pace the heart at a constant rate. But such cardiac pacing itself also alters AV nodal function, and this interacts with the vagal control of AV nodal function. There previously was no way to resolve this dilemma. We plan to use this technique to look afresh at pure vagal control of separate SA and AV nodal function. The second project concerns the effect of brief bursts of vagal and sympathetic activity, and the dynamic interactions of various patterns of such neural stimuli and heart period on AV conduction. The various patterns of stimuli will mimic the patterns most commonly seen in vivo. These should provide basic new insights into the physiology of the control of AV conduction. This information will directly bear on our understanding of many clinical arrhythmias where the important role of the autonomic nervous system is becoming more fully appreciated in recent years. This information is additionally important for the design and clinical optimization of artificial cardiac pacemakers where serious problems may now arise from our poor understanding of these phenomena.